Studv On The Solution Enitaxy,microstructures And Properties Of Perovskite Ferroelectric Oxide Heterostructures

Perovskite ferroelectric oxides have a spontaneous polarization and the polarization direction can be reversed with the applied electric field,affording broad applications in the fields from sensors,memory to photocatalysis.In recent years,perovskite ferroelectric oxide heterostructures have been a focus of materials science and condensed matter physics due to their novel physical and chemical properties.Therefore,the design and synthesis of perovskite ferroelectric oxide heterostructures as well as the systematic study of controllable solution epitaxial growth,interface microstructure,ferroelectric polarization screening mechanism and interface properties are of great scientific significance.In this dissertation,we firstly reviewed the crystal structure and functionality of perovskite oxides,summarized the polarization screening mechanism of perovskite ferroelectric oxides and the chemistry of ferroelectric surfaces.Meanwhile,the preparation methods,interface properties and corresponding physical mechanisms of perovskite oxide heterostructures,especially perovskite ferroelectric oxide heterostructures were discussed in detail.However,the effect of electrostatic force originating from a polar ferroelectric surface on the epitaxial growth,interface microstructure and properties remains unclear.The main issue is that the classical preparation methods,such as pulsed laser deposition(PLD)and molecular beam epitaxy(MBE),require a relative high synthesis temperature(generally higher than 600℃),resulting in a significant decrease or even disappearance of the ferroelectricity of the substrate.Accordingly,we innovatively proposed an idea of the electrostatic force-driven oxide heteroepitaxy,used the single-crystal and single-domain PbTiO3(PTO)nanoplates as the substrate,designed and synthesized various perovskite ferroelectric oxide heterostructures and systematically studied their interface microstructures,ferroelectric polarization screening mechanisms,interface properties and their possible physical mechanisms.The main innovative results are listed as follows:(1)SrTiO3/PbTiO3(STO/PTO)single-crystal heterostructures were successfully synthesized via a hydrothermal method and STO films with a thickness of 22 nm selectively grew on the positive polar surface of PTO nanoplates.STO/PTO heterostructures formed an atomically sharp interface with an epitaxial relationship of{001}STO//{001}PTO.Electronic screening via electrons within PTO itself and ionic screening from polarized STO contribute to the complete screening of positive polar surface of PTO in STO/PTO heterostructures.(2)A room-temperature ferromagnetism has been observed in STO/PTO heterostructures with a saturation magnetization of 0.002 emu/g.A reversible magnetic transition from ferromagnetism to diamagnetism could be induced by applied magnetic field.Meanwhile,the transition magnetic field increased with the decrease in temperature.Experimental and theoretical studies demonstrate that the ferromagnetism of STO/PTO heterostructures originates from the coupling between the Ti dxy localized moments and the conductive electrons at the interface.Conductive electrons would be localized with a applied magnetic field and form a antiferromagnetic coupling with Ti dxy localized moments.Therefore,STO/PTO heterostructures exhibited intrinsic diamagnetism of the bulk material.(3)BiFeO3/PbTiO3(BFO/PTO)single-crystal heterostructures were successfully synthesized via a hydrothermal method.BFO films selectively grew on the negative polar surface of PTO nanoplates with a saturation thickness of about 18-20 nm.BFO/PTO heterostructures formed an atomically sharp interface with an epitaxial relationship of {012}RFO//{001}PTO.The spontaneous polarization of BFO and the positively charged oxygen vacancies contribute to the full screening of negative polar surface of PTO in BFO/PTO heterostructures.The ferroelectric polarization and the charged precursor(Bi25Feo40)affects the selective growth of BFO films and the process of "screening balance broken and rebuilt" results in the saturation thickness of BFO films.The controllable thicknesses of BFO films were achieved by tuning the raw material concentration below the saturation thickness.(4)Similar to that of STO/PTO heterostructures,a room-temperature ferromagnetism was also observed in BFO/PTO heterostructures with a saturation magnetization of 0.01 emu/g.A reversible magnetic transition from ferromagnetism to diamagnetism could be induced by applied magnetic field.Meanwhile,the transition magnetic field increased with the decrease in temperature.The saturation magnetization of BFO/PTO heterostructures decreased with the decrease of BFO film thickness.(5)TiO2/PTO heterostructures were successfully synthesized via a hydrothermal method.It has been found that both the ferroelectric polar surface and pH of the solution determine the selective growth behavior of anatase TiO2.The modification of surface by mineralizer ions influences the morphology of the anatase TiO2.TiO2 films with{001} exposed selectively grew on the positive polar surface of PTO nanoplates using HF as the mineralizer;TiO2 octahedrons with {101} exposed grew on the both side of PTO nanoplates using H2O as the mineralizer;TiO2 octahedrons with {101} exposed grew on the negative polar surface of PTO nanoplates using NH3 H2O as the mineralizer.It should be noted that the epitaxial relationships of all the TiO2/PTO heterostructures were {001}s-tO//{001}PTO,forming an atomically sharp interface.(6)The growth mechanism of TiO2 was explained based on the surface energy and lattice matching.The atomic reconstruction and electronic reconstruction contribute to the complete screening of positive polar surface of PTO in TiO2/PTO heterostructures.(7)TiO2/PTO heterostructures adopt a polarization-dependent photocatalytic performance under visible light irradiation.For the hydrogen evolution reaction(HER)of water splitting,the H2 production rate of(TiO2/PTO)+ heterostructures was 2.43 times higher than that of(TiO2/PTO).heterostructures.However,for the photodegradation of MB,the reaction rate KMB of(TiO2/PTO).heterostructures was 9.5 times higher than that of(TiO2/PTO)+heterostructures.The DFT results show that the photogenerated electrons can be inj ected into the TiO2 only at the positive side interface and the photogenerated holes transfer from the PTO to TiO2 at the negative side interface,thus the interface band alignment caused by ferroelectric polarization results in the polarization-dependent visible-light catalytic performances.(8)For the first time,NaTaO3/SrTiO3(NTO/STO)heterostructures were successfully synthesized via a hydrothermal method using single-crystal STO(10 mm× 10 mm × 0.5 mm)substrates.A large-area NTO single-crystal film was obtained and the controllable thicknesses(15 nm,26 nm and 60 nm)of NTO films were achieved.The NTO/STO heterostructure has been determined to adopt an atomically sharp interface with an epitaxial relationship of{001}NTO//{001}STO.It has been found that NTO/STO heterostructures demonstrate anisotropic magnetic properties at room temperature.An obvious ferromagnetism has been observed in the NTO/STO heterostructure with a saturation magnetization of 0.19 memu/cm2 and a coercive field of 27 Oe when the magnetic field is perpendicular to the surface of the sample,while a diamagnetic signal has been observed in NTO/STO heterostructures when the magnetic field is parallel to the surface of the sample.